Electronically functional materials such as conductors, semiconductors and insulators have many applications in modern technology. In particular, these materials are useful in the production of microelectronic components such as transistors (e.g. thin film transistors (TFTs)) and diodes (e.g. light emitting diodes (LEDs)). Inorganic materials such as elemental copper, elemental silicon, and silicon dioxide have traditionally been employed in the production of these microelectronic components, whereby they are deposited using physical vapour deposition (PVD) or chemical vapour deposition (CVD) methods. Recently, newly developed materials and material formulations with conducting, semiconducting or insulating properties have become available and are being adopted in the microelectronic industry.
One such class of electronically functional materials is that of organic semiconductor materials. Another class is that of inorganic metal colloid formulations dispersed in liquid solvents. While the first example is a recently developed class of materials, the second example uses traditional materials in a recently developed formulation type. These materials and material formulations are associated with a number of advantages over the traditional materials when used for microelectronic device production. One such advantage is that these materials can be processed in a greater variety of ways, including solution processing where the material is dissolved in a solvent or dispersed as a colloid, and the resulting solution is used to manufacture e.g. microelectronic components. This is advantageous because solution processing is very cost-effective. In particular, a significant saving can be made in terms of start-up costs associated with setting up plants for producing microelectronic components when compared with e.g. silicon semiconductor processing facilities where there is a need for high capital investment in expensive production facilities.
One particularly promising technique for the processing of semiconductors to form microelectronic components, for example TFTs and LEDs, is ink-jet printing. This is because ink-jet printing conveniently allows relatively precise deposition of a semiconductor solution onto a substrate in an automated manner. It would be highly desirable to be able to produce microelectronic semiconductor components on an industrial scale by ink-jet printing conductor, semiconductor and insulator solutions onto a suitable substrate.
However, there are fundamental problems in carrying this out in practice. The key problem is that, in the production of microelectronic devices, it is generally necessary to produce high-resolution patterns of the electronically functional materials on a substrate. At present, ink-jet printing does not allow a high enough resolution to be achieved to allow the direct printing of suitable patterns onto a bare substrate. At present, there are two ways to avoid this problem.
The first way is to use photolithography to remove undesired areas of a blanket-deposited electronically functional material, very high-resolution patterns being obtainable by this method. However, photolithography is a subtractive technology and is expensive both in terms of initial investment in expensive photolithographic equipment and in terms of the relatively large number of processing steps associated with these techniques, energy consumption and wasted material.
A second way of circumventing the resolution problems associated with ink-jet printing of patterns of electronically functional materials on bare substrates is to form a pre-pattern on the substrate prior to deposition of the electronically functional material thereon which directs the ink-jet-printed solution onto specific areas. Generally, this involves treating the substrate to form a wetting contrast consisting of adjacent areas on the surface having different hydrophilicity and/or oleophilicity to ensure different interaction with electronically functional inks subsequently printed thereon. Thus a substrate can be produced having ink-receptive areas and ink-repellent areas, so that a droplet of ink landing on an ink-receptive area of the substrate would be prevented from spreading onto the adjacent ink repellent area. Similarly, any droplet of ink landing so that it contacts both the ink-receptive and ink-repellent areas would be pushed towards the ink-receptive areas. In this way, the resolution of an ink-jet printer can be enhanced to allow the required resolution to produce patterning as required in the production of microelectronic devices. For this to work effectively, the difference in hydrophilicity and/or oleophilicity between the two areas of the substrate should be as large as possible.
At present, this latter technique requiring the establishment of adjacent ink-receptive areas and ink-repellent areas on a substrate has only been realised on inorganic substrates such as indium tin oxide or silicon oxide (glass) plates. Where such a substrate is used, it is conventional to apply a photo-crosslinkable polymer (=negative resist) coating (for example polyimide) to an inorganic oxide plate and then selectively dissolve those parts of the polymer coating that were protected by a photomask against the UV-irradiation during a crosslinking step to reveal the underlying inorganic oxide. Subsequent treatment of the entire substrate with e.g. a CF4 plasma leaves the exposed inorganic oxide substrate hydrophilic but renders the polymer surface hydrophobic and oleophobic thus establishing a wetting contrast. Subsequent printing of an aqueous conductor ink onto the exposed glass parts allow a high resolution pattern to be formed even if the patterning carried out is required to be of higher resolution than the ink-jet printing because droplets of aqueous ink falling in part on the hydrophobic and oleophobic polymer area will be pushed on to the hydrophilic glass area.
Whilst this method of creating adjacent ink-receptive and ink-repellent areas on the substrate is generally quite effective in increasing the resolution obtainable when ink-jet printing a solution of an electronically functional material, significant problems are associated with these techniques when carrying them out on a commercial scale.
In order to reduce production costs, it is desirable to print microelectronic devices using a so-called reel-to-reel (R2R) production environment. Here, a substrate is rolled off a first reel, processed, and then rolled onto a second reel. A precondition for using such a production method is that the substrate must be flexible. At present, the flexible substrates of interest are most often polymer foils. However, none of the flexible substrates which are currently available are suitable for making substrates with appropriate wetting contrasts in a commercially viable manner.
It is possible to produce a wetting contrast on a polymer substrate, for example by exposing one part of the substrate to O2 plasma to render it hydrophilic and to expose another part to CF4 plasma to render it hydrophobic and oleophobic. However, CF4 treatment affects both pristine polymer surfaces as well as surfaces which have been exposed to O2 plasma, so that surface patterns which are to remain hydrophilic after CF4 treatment must be protected by a photoresist mask during the CF4 plasma treatment. This is not desirable, in part because this requires two extra processing steps (the application and removal of the mask) which adds to the production cost, but mainly because the hydrophilicity of the hydrophilic area is decreased on removal of the mask due to residual photoresist material which cannot be removed. An inversion of the order of the processing steps might in theory alleviate the latter of these problems, but cannot be realised as photoresist material does not adhere to a fluorinated surface. Therefore, it is not possible to produce flexible substrates with a wetting contrast where the adjacent areas making up the contrast area differ enough in hydrophilicity and/or oleophilicity for these to be used to good effect in ink-jet printing solutions of electronically functional materials onto these to produce microelectronic components.
Accordingly, there is still a need for the realisation of wetting contrasts on flexible polymer foils to allow an increase in the resolution of ink-jet printing electronically functional inks onto such substrates to produce microelectronic devices such as TFTs and LEDs.
The present inventors set out to provide a commercially useful method of producing a substrate having an appropriate wetting contrast wherein the substrate is not limited to being a rigid substrate formed from an inorganic oxide such as glass or indium tin oxide, and wherein the above-mentioned problems can be avoided.